[I'm cross posting it to g.e.help because I explicitly mentioned this
package in a message a little while ago]

semantic-hover-completion allows you to get a navigable pop-up
completion, much in the IDE vein; it tries pretty hard to be
completely transient and never get in your way. It uses Semantic
Bovinator to do the dirty job of actually finding completions, so they
tend to be sensible and syntactically and semantically correct.

You can use it in two basic ways: by electric strings, (see
semantic-hover-completion-add-electric-string, which is designed to be
run in mode hooks), or running (preferably by binding to a key)
semantic-hover-completion-popup. In either case, you get a tooltip with
available suggestions, navigable by keyboard. In case you change your
mind and decide to do something else than play with completions, all
your normal keybindings should work, and testing shows they usually
do. One notable exception is bindings involving , as it's a
translated event and exposes a very annoying bug in Emacs API. You
might have better luck with substituting DEL instead, but no
guarantees.